. >> reporter: hitting the fast food drive-in for science. >> can i get five soft tacos? can i get a number four? >> reporter: over and over and over again, it has been dave giocolo's life the past few months or so. as has been eating the food. a precisely calculated 1,000 calories a day. how many calories? 770. >> reporter: this man is paying them to. dr. samuel klein is a researcher at washington university in st. louis. trying to understand why fast food leads to weight gain and diseases like diabetes and hypertension. at some point, though, research on rats alone is not enough. somebody has to eat the food? >> someone has to do it. >> reporter: and so, a radio ad, that offered a cash incentive, up to $3,500, depending on how long it would take. >> i called right away. >> reporter: so did nurse, dawn freeman. >> it took a month. >> reporter: to get approved? >> to get approved. >> reporter: so, they ate every day, from one of these five restaurants, until their weight went up 5% or 6%. dawn, in eight weeks went from 170 to 186. doesn't look like a lot. but here's how i